Our present understanding of the in vivo dynamics of folate is inadequate in light of the importance that folate plays in the pathogenesis of many diseases. The marginal to poor folate nutrition status of at least one in ten Americans is associated with several chronic and developmental diseases that include neural tube defects, cancer, and homocysteinemia, an independent risk factor for coronary heart disease. The mechanisms behind the pathology are believed to involve a dysfunction in the dynamic and kinetic behavior of folate metabolism; accordingly, quantitative information on the in vivo metabolism of folate has emerged as a high scientific priority. New isotope tracer methodologies employing radiocarbon tagged folates coupled with Accelerator Mass Spectrometry (AMS) detection promise to revolutionize in vivo tracer studies in humans. Using this approach, [14C]folate is reliably detected at attomole concentrations (moles x 10-18) in plasma, erythrocytes, urine and feces up to 200 days following a single, physiologic (35 mug) oral dose. At these levels of sensitivity, doses are virtually non-radioactive, permitting testing in both healthy subjects and at-risk subpopulations for folate-dependent disease. Our long-range goal is to understand the dynamics of human folate metabolism in terms of known hereditary and environmental factors that modulate incidence and progression of folate-related diseases. Relevant examples include how derangements of the genetic material, such as the common methylenetetrahydrofolate reductase (MTHFR) gene polymorphism (a key folate metabolizing enzyme) may lead to homocysteinemia, or how pregnancy effects the mobilization and utilization of body folate stores. In pursuit of this goal, we propose long-term (7 month) tracer studies using [14C]folic acid and AMS detection to define the kinetics of folate metabolism in healthy female and male subjects. These investigations will fill a critical knowledge-gap surrounding folate metabolism. A kinetic approach offers a precise mode of quantitating the relative importance of absorption, distribution and elimination in the individual response to folic acid. Analysis of kinetic data will facilitate the construction of kinetic models and the establishment of metabolic phenotypes; this information will serve as a reference point for future investigations within at-risk sub-populations and individuals.